Tire Size

[BigSur]

Originally Posted by JackandJanet

Your truck speedometer is an analog device. It receives digital data from the PCM, the same that is fed to the Gryphon and the digital odometer. In the conversion to analog, mechanical gear/spring errors cause the difference. I suspect that Ford insured the speedometer would read slightly fast so they could never be sued for someone getting a speeding ticket due to a speedometer reading slow. That's a guess, of course, but if there has to be an error, I'd expect Ford to "bias" it in that direction. Because mechanical devices can't really be dead accurate, speedometers in different vehicles are going to have slightly different errors.

But, there's no "conversion" for the odometer and the Gryphon speedometer. You should be able to get them to tell you something that is very close to the truth.

However, all this is a bit like counting angels on the head of a pin. Variations in tire construction, tire height/width profiles, tire pressure, vehicle loading, tire "slippage", and vehicle speed will ALL effect the accuracy of both the digital and analog reading. The PCM essentially counts axle revolutions, through a sensor. It then "calculates" speed and distance based on the number of revolutions it expects the axle to make in a mile (which the Gryphon inputs after calculating it from your tire circumference (or tire diameter if you have a CS/CTS). You should be able to get close, probably within 0.1%, but much more than that is just wishful thinking. (0.1% is 1 mile in 1000 miles).

As shotgun said: "Welcome to the unofficial (but highly popular) TS Calculation Club." I'll add, "No matter what you enter, you'll be wrong". But, it sure is fun!

- Jack

Well, as always......thanks very much for the help. Without the forums and members like you guys who are willing to help those in need, many of us would still be sitting around confused w/ tons of unanswered questions. With that said, thanks again.

I'm going to bump it up to 2575 today and see what I get and will then continue to adjust as necessary.